U.S. Pat. No. 6,497,094 discloses a method and an arrangement wherein a signal for the secondary air flow is determined and an evaluation of the operability of the secondary air system is based on this signal. The signal for the secondary air flow is determined from the following: a signal for the intake air flow; a signal for the ratio of the intake air flow to the fuel metered per unit of time which is determined by computation; and, an exhaust-gas lambda value detected by means of a probe.
In order to further increase the accuracy in the determination of the signal for the secondary air flow, the secondary air flow could be measured directly with an air mass sensor which, for example, is mounted on the secondary air pump. The result of this measurement (actual value) is compared to a desired value. For a deviation of the actual value from the desired value, the pump power of the secondary air pump is so readjusted that the desired and actual value become coincident. What is problematic is that faults, which affect the operability of the secondary air system, are only detected when the control range of the secondary air pump is exceeded. Faults which can affect the operability of the secondary air system include, for example, a defective air sensor or leakages or obstructions in the secondary air system.